There are many potential military and commercial applications for an improved (i.e., longer-range and more economical) passive magnetic sensing system that can detect, track and measure the DC magnetic anomaly fields of moving magnetic objects or “targets” in real-time. The word “passive” indicates that the magnetic sensing system does not produce magnetic anomaly fields but only detects (and processes) the magnetic anomaly field that emanates from a target's inherent magnetic signature. The magnetic signatures stem from ferrous materials that are contained in the physical structure of a target.
Targets of interest that produce detectable magnetic signatures include watercraft such as naval vessels, and land vehicles such as cars, trucks or military tanks. Frequently, the presence, location, state of motion, and magnetic signature of these targets must be determined. Different types of targets typically will have different magnetic signatures that can be correlated with the target's ferrous structure. Thus, measurements of a target's magnetic signature can be used to classify the target. However, the physical nature of magnetic anomaly fields (i.e., a rapid reduction in magnetic field strength with distance) has limited the effective range of current magnetic anomaly sensing-based systems.
Currently, point-by-point “detection, localization and classification” (DLC) of magnetic objects generally requires a number of vector magnetic sensors that are configured as magnetic gradiometers. A gradiometer measures magnetic gradients, i.e., the rates of change of magnetic fields with distance. It is known in the art that passive magnetic detection and ranging of moving targets can be achieved by using a stationary magnetic sensing system having a combination gradiometer/magnetometer that measures five independent gradient tensor components and at least one vector field component of the target's magnetic anomaly field. However, because of the limitations of conventional prior art approaches with regard to their sensing system embodiments and signal processing methods, they have not produced a practical long-range DLC and/or tracking system.
Several advancements in magnetic-based DLC and tracking are disclosed in U.S. Pat. No. 7,342,399 (i.e., “the '399 patent” as it also will be referred to hereinafter) where a novel gradiometer-based system for tracking a magnetic object is disclosed. The '399 patent teaches an improved magnetic anomaly sensing-based system for tracking and classifying magnetic objects. In particular, the '399 patent describes a magnetic anomaly gradient sensing system based on the teachings in U.S. Pat. Nos. 6,476,610 and 6,841,994.
U.S. Pat. No. 6,476,610 (i.e., “the '610 patent” as it will also be referred to hereinafter) disclosed a novel magnetic gradiometer and signal processing concept denoted as “Scalar Triangulation and Ranging” (STAR) for target localization from maneuverable sensing platforms. The prior art STAR concept uses unique, rotationally invariant scalar “contractions” of magnetic gradient tensor components to “triangulate” relative distances to a target. Within the target-detection distance of a STAR-type gradiometer, the scalar triangulation process does not directly depend on the target's magnetic dipole signature. Thus, a STAR-type sensing technology can track a magnetic target even as its magnetic signature changes due to the target's motion in the Earth's magnetic field.
U.S. Pat. No. 6,841,994 (i.e., “the '994 patent” as it will also be referred to hereinafter) disclosed significant improvements to the STAR gradiometer design and method that better determine the range, relative bearing and magnetic signature of a stationary target from a mobile sensing platform. The '399 patent discloses a unique application of the '994 patent's magnetic gradient based STAR method that can be used to detect a moving magnetic object, and accurately determine the object's position and changes in its position, velocity and magnetic moment signature while compensating for the aspherical nature of the magnetic object's gradient contraction contours. The '399 patent also discloses a magnetic anomaly sensing system that can be used to remotely align or point an external device or system at a moving magnetic object. Thus, the '399 patent disclosed an improved magnetic gradient sensing based STAR technology that can overcome the limitations of prior art technologies and detect, track and measure the DC magnetic anomaly fields of moving magnetic objects or “targets” in real-time.
The tracking technology disclosed in the '399 patent is a relatively short-range technology because it uses magnetic gradient based methods to track and classify magnetic targets. Note that all magnetic gradiometer based sensing systems are relatively short-range systems since magnetic gradient signals are proportional to the inverse fourth power of distance from a target and very rapidly decrease to a sensor system's noise level as sensor-to-target distance increases. In addition, STAR gradiometer-based sensing systems generally require seven or eight “triaxial magnetometer” (TM) type vector magnetic field sensing elements. While these multiple-TM arrays enhance the effectiveness of a STAR-type gradiometer, they also increase the physical size, complexity and cost of the technology and detract from its practical usage.
Recently, U.S. Pat. No. 7,932,718 disclosed a magnetic anomaly sensing system and method using at least four triaxial magnetometer (TM) sensors. Each TM sensor has orthogonal X, Y, Z magnetic sensing axes such that the basic four TM sensor system produces 12 vector magnetic field (or “B-field” as it is known) equations, i.e., four BX equations, four BY equations, and four BZ equations. The system of 12 vector equations are readily used to solve for 6 unknowns, i.e., the magnetic signature M along each of X, Y, Z axes and the X, Y, Z coordinates of the target generating the magnetic signature. The excess number of available independent equations relative to the number of unknowns supports the use of a conventional least squares minimization for the non-linear X, Y and Z followed by a linear solution for the magnetic signature components MX, MY and MZ. The non-linear squared and cubic powers of the X, Y and Z terms in the classical vector equations produces several solutions at each TM sensor. However, some of the solutions are undesirable because they can occur in a mirrored quadrant where there is no target and other solutions are undesirable because they are imaginary (complex). While arranging at least four TM sensors in different planes of reference combats the above problems, it does so at the expense of system size, weight and cost.